A tap changer is an electrical switch customarily utilized with transformers to obtain a voltage ratio which is slightly different from the standard ratio obtained with the complete high voltage winding. A plurality of contacts in the tap changer are connected to taps in the high voltage winding of the transformer. Tap changers usually have five positions or steps in order to allow a change of either two steps above and two steps below or four steps below the rated voltage. These steps are usually 2.5% of rated voltage. The switching action is obtained by shorting two adjacent contacts and thus changing the number of turns of the high voltage winding of the transformer. The switching action is, preferably, done while the transformer is de-energized.
The prior art tap changers have been relatively complicated electromechanical devices which have been expensive to manufacture and difficult to assemble. The tap changer contacts have typically had relatively thin upper walls, preferably crimped to a cable, and thick bottom walls for strengthening purposes. The manufacture of these contacts has been relatively expensive because of this construction.
Prior art tap changers have also had difficulty with the problem of tracking. Tracking is the conduction of electricity between two dielectric substances from two different voltage levels. This tracking reduces the life of the tap changer. A further difficulty with prior art tap changers has been the necessity of using one tap changer which may be mounted inside the transformer cabinet while requiring the utilization of a separately designed tap changer which is mounted externally of the transformer cabinet.
U.S. Pat. No. 3,652,812, discloses a tap changer switch having a radially directed pressurized movable contact assembly. The switch of Ristuccia is, however, a very complicated structure which is relatively expensive to manufacture and difficult to assemble. The tap changer of Ristuccia requires the utilization of a double shaft assembly. Assembly of the tap changer is complicated by the orifices through which the shafts pass.